Looking Up: Look for the Zodiacal Light

Wisps of sheer luminance tangled among the dusting of faint stars await the keen-eyed or fortunate observer who can get away to a dark night sky.

Peter Becker

Wisps of sheer luminance tangled among the dusting of faint stars await the keen-eyed or fortunate observer who can get away to a dark night sky. The celestial dome over our heads is meant to be spangled with stars, speckled here and there with brilliance but overwhelmed by faint infusion of less bright stars, nebulae, zodiacal light and the majesty of the Milky Way Band.

All too often we must be content with a sky compromised by light pollution. Fortunately you still enjoy much of the sky not far from the cities, and even from the city you can see the bright stars, planets and moon. Be thankful if you have a good, dark rural sky, or can travel to see it.

From a reasonably dark site, when the moon is not around, look for the faint tapestry of light that joins the starry host. It has been said that most Americans have never seen the Milky Way Band. This is not surprising, and it isn’t just because most people live in congested areas where there is more light pollution. One goal of this column is to inspire more people to consider time away from the TV and computer screen to see the sky from their own back yard. If the sky is good and clear, prepare by letting your eyes adjust to the dark for a few minutes. Then look up and see if you can see the dim, billowing Milky Way Band. While more prominent in the summer evening sky, you also can see the band in the winter. In the evenings of late February and early March, the Milky Way stretches from the south-southeast, passing overhead and then down in the northwest. The bright winter constellation Orion, marked by its famous “belt” of three stars, is immediately right of the Milky Way Band. Overhead, the bright yellow star Capella is right in the Milky Way.

Scanning this area with binoculars will reveal the band is full of faint stars and star clusters, a revelation first made by the astronomer Galileo, who first studied the sky with a telescope in the early 1600’s. Users of small and larger telescopes could spend hours exploring this star-rich region of our home galaxy.

The Zodiacal Light is about as bright as the Milky Way Band. This phenomena is much closer to home, being a cloud of tiny dust particles encircling the sun, spread beyond Earth’s orbit and in the same plane as Earth and other planets. It is dimly visible from a dark site, just after evening twilight in the west, or before morning twilight in the east. Late winter and early spring evenings are favored for seeing the evening Zodiacal Light in the Northern Hemisphere. This is because the narrow path of the planets, known as the “ecliptic” and sometimes as the “zodiac,” is steeper with respect of the west horizon this time of year. The Zodiacal Light then “stands up” the straightest, further away from the horizon where Earth’s atmosphere is thickest. Stars, the Milky Way and Zodiacal Light are much harder to see near the horizon, even at a dark site; in addition, any light pollution from a distant town will wipe it out close to the ground.

Note that the Zodiacal Light appears as a dim cone of light, broader near the bottom. The writer has not yet had the opportunity to see this phenomena from home, given the amount of light pollution, but he did have a wonderful view a few years ago. He saw it from some 30,000 feet, in a west-facing window seat of an airliner coming back from Florida in March. With a jacket pulled over his head, he could plainly see the cone of light reaching up and was surprised at how bright it was.

Much harder to see than the Zodiacal Light is the continuation of this glow, which stretches completely around the sky. At a very dark site, at around midnight and when the ecliptic is high up, it is possible to barely detect a slight brightening of this ghostly band of solar system dust, a fuzzy oval of light known as the gegenschein. Ideal conditions are needed to successfully detect it. The Milky Way Band must not be in the vicinity, as this is enough to hide the overlapping gegenschein. Around midnight in late winter may be a good time, if you look about half way up in the south. This oval is slightly more luminous than the rest of the almost invisible band, because it is directly opposite the sun and each dust particle is facing you in “full phase” in the same way the moon is brightest when full.

The new moon is on March 7, so until then, we will have darker, moon-less evening skies, to enjoy the stars and the dimmer cosmic realm.

Keep looking up!

Peter W. Becker is managing editor at The Wayne Independent in Honesdale, Pa. He has been an amateur astronomer since the age of 12, in 1969. He may be reached at pbecker@wayneindependent.com.